The Constitution.
We actually have a different word for things that violate the Constitution ("unconstitutional"). Ron Paul said the Iraq war was "illegal" and like I said, you can't tell me what law it violates. Apparently the only way you can think of to defend Paul's words is to try to pretend Ron Paul said something different from what he actually said and hope I don't notice.
As for it being unconstituional there is nothing unconstitutional about the Iraq war or the War Powers Act. If you think there is, kindly cite the Supreme Court case where "the Iraq war" or the War Powers Act was declared unconstitutional. Because failing that all we're left with is self-anointed Supreme Court Justice-at-large Ron Paul shooting his mouth off.